Method of welding stainless steel



Patented Jan. 27, 1953 UNITED STATES PATENT OFFICE METHOD OF WELDING STAINLESS STEEL of Delaware No Drawing. Application October 23, 1950, Serial No. 191,726

3 Claims. (Cl. 29358) The present invention is directed toward a method for welding ferrous alloys comprising substantially chromium, iron and nickel and referred to in the trade as stainless steels. More specifically, the invention is directed toward the prevention of corrosion due to welding of stainless steels which frequently manifests itself as an after effect when the welded stainless steels are employed.

This application is a continuation-in-part of Serial No. 716,669, filed December 16, 1946, now abandoned, for Prentiss S. Viles and entitled "Method for Welding Stainless Steel."

The chromium-nickel stainless steels are those containing nickel and chromium plus other elements to a lesser degree. These chromium-nickel stainless steels are known as the austenitic chromium-nickel stainless steels, the nickel being added in sufiicient amount to make the steels austenitic and nonmagnetic with improved ductility and conductiveness over the straight chromium steels.

These austenitic steels are outstanding in their resistance to corrosion and oxidation. Because of their excellent physical properties both at low and elevated temperatures they find wide application in the food, pulp, paper, textile, dye, chemical, oil refining and other related industries. The severe corrosion encountered in the use of these alloys, however, introduces many perplexing problems and many uses will demand a selection among the various types of steel available.

The corrosion characteristics of the steel are afiected not only by the media in contactwith the structural material but by the treatment the steel is given in its fabrication into structural shapes and facilities. In fabricating containers, pipes and various structural shapes and equipment it is necessary to employ welding. During the welding operation the stainless steel may be subjected to temperatures between 800 and 1600 F. whether the welding be accomplished by the oxy-acetylene, atomic hydrogen, or by the electric are methods. When the austenitic chromium nickel steels are subjected to temperatures between 800 and 1600 F. as occurs during welding. a structural change is effected in the steels. The carbon migrates out from the solid solution and combines with the chromium to form chromium carbides along the grain boundaries. This effect is known as carbide precipitation and this action impoverishes the chromium content adjacent to the 'grain boundaries; this increases the susceptibility to corrosion attack and leads to what 2 is generally referred to as intergranular corrosion at the grain boundaries.

The precipitation of carbide is affected by a number of factors among which is the time the material is held within the critical temperature range. The longer the heat treatment the more carbides precipitate and as the temperature approaches 1200 F. the more undesirable will be the structural change in the steel. It follows that the higher carbon content steels are more susceptible to carburization since there is more carbon available to combine with the chromium.

The problem of carbide precipitation and corrosion by intergranular attack has been solved in the industry by subjecting the welded metal toa heat treatment. The heat treatment involves heating the metal to a temperature be-' tween 1900 and 2000 F. for a sufiicient time to allow the chromium carbides to redissolve and go back into solution. Following the heat treatment the steel is quickly cooled by puenching with water or by air blasts or other suitable means. It is necessary to cool the heat treated material rapidly through the temperature range between 1600 and 800 F. to avoid the possibility of carbides forming again in passing through the critical temperature region.

Another method of suppressing the formation of chromium carbides in welding involves addition of several of the rare metals such as colum him and the like to the metals to act as stabilizers. These materials combine with the carbon and prevent or retard the formation of objectionable chromium carbides and thus reduce the tendency to intergranular corrosion.

The prior art methods in overcoming the formation of chromium carbides in the welding of stainless steels is expensive in that furnaces for the heat treatment must be provided which may also contribute to the cost of the steel. Too, even with the addition of stabilizer metals, carbide formation may be encountered.

Itis, therefore, the main object of the present invention to provide a method of welding austenitic stainless steel alloys which will substantially reduce or prevent the precipitation of chromium carbides at the grain boundaries of the metal and thus reduce the susceptibility of the welded material to intergranular corrosion.

In accordance with the present invention the precipitation of carbides during welding operations with its attendant degradation in the quality of the steel is materially reduced or substantially eliminated by the employment of a retarding agent during the welding operation. The carburization retarding agent employed in the present invention is a combustible sulfur-containing compound which is employed in the welding gas when oxy-acetylene or other gaseous media is used for supplying the heat.

The sulfurcontaining compound useful in the present invention may be hydrogen sulfide, the volatile organic sulfides such as carbon disulilde, the mercaptans such as ethyl mercaptan, and the like. As mentioned before the sulfurcontaining material is added to or blended with the gas employed during the welding operation.

It is contemplated that the amount of sulfurcontaining compound employed will vary from about 3% to 25% by volume added to the gas employed in the welding operation. Hydrogen sulflde in amounts of about 5% and 25% in the gas has been found to give good results.

When other sulfur-containing compounds besides hydrogen sulfide are employed, such as carbon disulfide and ethyl mercaptan, it will be necessary to adjust the amount of sulfur-containing compound in the hydrocarbon fuel gas in view of the difference in molecular weight. Thus, while 5% by volume hydrogen sulfide is effective, the amount of carbon disulfide used should be no less than about 3% by volume and when ethyl mercaptan is used the minimum amount should be about 3% by volume of the mixture, following Avogadros hypothesis. Actually from the molecular weight of sulfur in the hydrogen sulfide, it will be seen that the sulfur present in the welding region when 5% by volume of hydrogen sulfide is employed will be approximately 4.7%. The amount of sulfur in the region of the adjacent boundaries of metals being welded may be only a small proportion of the sulfur-containing compound which is present in the welding gas and may be as little as 1% or less.

The sulfur-containing compound, when hydrogen sulfide is employed, may be blended with the hydrocarbon gas prior to employment of same in welding operations, or the blend may be made by admixing a stream of hydrogen sulfide with a stream of hydrocarbon gas and feeding the combined streams into the welding torch. When liquid sulfur-containing compounds, such as carbon disulflde and ethyl mercaptan, are used the liquid sulfur-containing compound may be placed in a vessel and the hydrocarbon gas bubbled through it to pick up a sufficient quantity of the sulfur-containing compound in the range given. It may be desirable to form an enriched mixture of gas and sulfur-containing compound by bubbling the gas through the liquid sulfur-containing compound and then diluting the gaseous mixture by adding further amounts of hydrocarbon thereto. Carbon disulfide and ethyl mercaptan are completely miscible with either acetylene or propane and the other hydrocarbons mentioned hereinafter in all proportions. Therefore, the desired quantity in the welding gas may also be obtained by simply mixing under pressure and then warming the container so that the desired amount would vaporize with the gas. Raoults law applies.

In order to illustrate the effectiveness of this process, three separate oxy-acetylene welds of the same type of 18-8 steel alloys were made. In the first weld conventional technique was employed. In the-second and third welds conventional technique was employed except that 5% and 25 volume per cent of hydrogen sulfide were used with the heating gases. These welded strips were then tested for intergranular corro- Table volume Straus Test, 72 Hrs. Percent Metal Welded H 8 in Metal Welding Lo Condition Alter Bend- Percent 18-8 T 103 0 l. 97 Broken granular cracks.

be??? 5 0.310 Good, no cracks. Do 25 0. 48 Fair, very few cracks.

1 American Iron and Steel Institute.

The data presented in the foregoing table show that the employment of hydrogen sulfide during the welding operation effectively inhibits loss of metal when the welded specimen is subjected to a boiling Straus solution. Furthermore, the data show that the stressing of the tested sample produced less detrimental effect in the material wedled in accordance with the present invention than in the untreated specimen.

The Straus test results indicate that hydrogen sulfide in the amounts used inhibits carbide formation and precipitation of crabides at the grain boundaries and therefore substantially reduces susceptibility to intergranular corrosion.

The data also show that 5% H28 in the welding gas gives very satisfactory results and that while 25% gives a good weld evidence of some carbide formation is present. It is, therefore, undesirable to employ in excess of 25% H28 in the welding gas. Less than about 5% H28 is an impracticable amount to use due to difficulties in blending hydrogen sulfide with the hydrocarbon gas and obtaining proper concentration in contact with weld metal.

The mechanism of the process is not fully understood; however, it is postulated that sulfurcontaining compounds in the welding region inhibit metallic carburization and metal carbide formation; thus, chromium carbide is never formed in the process and therefore there are no chromium "lean areas in the alloy after welding. This allows the alloy to retain its original corrosion resistant characteristics.

The welding fluxes employed are the welding fluxes commonly used in welding stainless steels. Usually these welding fluxes are composed of some compound of the silicates, oxides, hydroxides, halides, carbonates, etc. of magnesium, calcium, sodium, potassium, lithium, and other alkali or alkaline earth metals along with small amounts of salts of titanium, manganese, columbium, vanadium, etc.

The gases employed in the welding operation may be acetylene or the liquefied petroleum gases such as propane, butanes, butylenes, propylene, mixtures thereof, and the like.

The nature and objects of the present invention having been fully described and illustrated, what I wish to claim as new and useful and to secure by Letters Patent is:

1. A method of welding a chromium-nickel austenitic stainless steel which includes the steps of forming a mixture of a hydrocarbon gas suitable for welding operations in an amount in the range between about 95% and 75% by volume and hydrogen sulfide in an amount in the range between about 5% and 25% by vol me, igniting said mixture to produce a flame sufllc ently hot to heat adjacent boundaries of bodies of said alloy to a fusion temperature, applying said flame to adjacent boundaries of said bodies and fusing said adjacent boundaries of said bodies with said ignited mixture.

2. A method of welding a chromium-nickel autenitic stainless steel which includes the steps of forming a mixture of a hydrocarbon gas suitable for welding operations in an amount in the range between about 97% and 75% by volume and a combustible sulfur compound in an amount between 3% and 25% by volume, igniting said mixture to produce a flame sufliciently hot to heat adjacent boundaries of bodies of said alloy to a fusion temperature. app ying said flame to adjacent boundaries of said bodies and fusing said adjacent boundaries of said bodies with said ignited mixture.

3. A method of welding a chromium-nickel autenitic stainless steel which includes the steps of forming a mixture of a hydrocarbon gas suitable for welding operations in an amount in the range between about 97% and 75% by volume and a combustible sulfur compound selected from PRENTISS S. VILES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 249,393 Patten Nov. 8, 1881 FOREIGN PATENTS Number Country Date 23,779 Great Britain 1908 1,403 Great Britain 1909 OTHER REFERENCES "Alloys of Iron and Chromium by Kinzel and Franksvol. 2, page 181 (103, Welding) published by McGraw-Hill Book Co., New York, N. Y. (Copy in Div. 3.) 

1. A METHOD OF WELDING A CHROMIUM-NICKEL AUSTENITIC STAINLESS STEEL WHICH INCLUDES THE STEPS OF FORMING A MIXTURE OF A HYDROCARBON GAS SUITABLE FOR WELDING OPERATIONS IN AN AMOUNT IN THE RANGE BETWEEN ABOUT 95% AND 75% BY VOLUME AND HYDROGEN SULFIDE IN AN AMOUNT IN THE RANGE BETWEEN ABOUT 5% AND 25% BY VOLUME, IGNITING SAID MIXTURE TO PRODUCE A FLAME SUFFICIENTLY HOT HEAT ADJACENT BOUNDARIES OF BODIES OF SAID ALLOY TO A FUSION TEMPERATURE, APPLYING SAID FLAME TO ADJACENT BOUNDARIES OF SAID BODIES AND FUSING SAID ADJACENT BOUNDARIES OF SAID BODIES WITH SAID IGNITED MIXTURE. 